There is a strong interest in using Metal Oxide Thin Film Transistors (MOTFT) as the active matrix backplane in digital X-ray imagers because of the higher mobility and stability of the MOTFTs. The high performance MOTFT enables an active pixel sensor (APS) configuration similar to silicon wafer based CMOS imagers. This is not possible with an amorphous silicon (a-Si) TFT backplane because of its poor mobility and Vth instability under operation. There is also a strong interest in having a flexible or conformable X-ray imager that can fit with the contours of the object being X-rayed. The key issue for such imagers is the MOTFT's compatibility with the X-ray image detector fabrication process.
One of the most popular X-ray image detectors is the X-ray scintillator/a-Si TFT/a-Si PIN photodiode combination. The X-ray scintillator converts the X-rays into visible light which is then detected by the a-Si PIN photodiode in an image sensing pixel of a two dimensional visible imager. Each sensing pixel comprises a switch transistor made of an a-Si TFT, a storage capacitor and a sensing element made of an a-Si PIN photodiode. Such pixel circuit is often called a passive pixel sensing (PPS) circuit. This combination is preferred because of its simple and inexpensive construction. In high pixel density and small pixel pitch designs, it is preferred to have the PIN photodiode plane on top of the TFT backplane so that the metal lines and transistors in the backplane will not block visible light coming from the X-ray scintillator. But unfortunately the fabrication of the a-Si PIN photodiode plane on top of (i.e. subsequent to) a MOTFT backplane can be an issue. Primarily, the MOTFT backplane will have a difficult time surviving the high temperature and the ambience of a-Si fabrication. In addition to a-Si based pin photodiodes, almost all high performance PIN photodiodes made with inorganic semiconductor materials are deposited at high temperatures and/or need post annealing at high temperature. The issue above is thus a general issue for an X-ray imager with a MOTFT-based back-panel circuit.
Because of this problem, the fabrication process is constrained to form the a-Si PIN photodiode plane first and then form the MOTFT backplane on top of the a-Si photodiode plane. The key problem in this orientation is that the metal lines and transistors in the MOTFT backplane will block a significant portion of the incident light from the top mounted scintillator. The fill factor (the ratio of sensing area to the total pitch area) for MOTFT backplanes is very poor. Also, the fill factor issue gets worse for high resolution imager arrays because of the smaller pixel size (i.e. higher pixels density). Another motivation for using MOTFT backplanes for digital X-ray imagers is to use the active pixel sensor (APS) pixel readout configuration which provides signal amplification at each imager pixel site. However, in APS configurations more metal lines and transistors are needed compared with passive pixel sensor (PPS) configurations. Thus, the fill factor is even worse for APS than for PPS. Furthermore, if circuit compensation techniques are used to compensate for stability and uniformity, more transistors and control lines are needed, which makes the fill factor problem even worse. Thus, it is very difficult to make a high resolution APS X-ray imager using the MOTFT backplane and PIN photodiode plane combination.
In a copending U.S. patent application entitled “Pixelated Imager with MOTFT and Process”, filed Dec. 13, 2012, bearing Ser. No. 13/713,744, and incorporated herein by reference, the fabrication of rigid sensors is disclosed. Further, a wide variety of different sensors and displays are described.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved process for fabricating an X-ray image detector.
It is another object of the present invention to provide a new and improved process for fabricating an X-ray image detector incorporating the X-ray scintillator/PIN photodiode combination with a MOTFT backplane.
It is another object of the present invention to provide a new and improved process for fabricating an X-ray image detector with an active pixel sensor (APS) configuration.
It is another object of the present invention to provide a new and improved process for fabricating a flexible X-ray image detector.
It is another object of the present invention to provide a new and improved X-ray image detector incorporating an X-ray scintillator/PIN photodiode combination with a MOTFT backplane.
It is another object of the present invention to provide a new and improved flexible X-ray image detector incorporating an X-ray scintillator/PIN photodiode combination with a MOTFT backplane and an active pixel sensor (APS) configuration.